32-bit applications in a 32-bit OS can address (by default - unless you change some settings in windows) 2GB of RAM. The same 32-bit applications (if written well) in a 64-bit OS can address close to 4GB RAM.
So, there is some benefit to running x64, even if you only use 32-bit apps.
I would say it the other way around. You would not see a benefit in running 32-bit OS. Go for 64-bit if your hardware has 64-bit drivers all the way.
If you're going to upgrade, be aware that a direct upgrade from 32-bit to 64-bit is not possible. You can do easy transfer (it's on the windows 7 disc somewhere) to transfer all your files and settings to an external disk. Then install your 64-bit windows, and do easy transfer to transfer everything back.